Resuscitator/ventilator devices of this type, hereinafter termed, shortly, "gas-powered resuscitator/ventilator devices" typically include a control unit that generates the required pulses of breathable gas, and a so-called "patient valve" that is connected to the control unit by a flexible conduit or hose and that is associated with an oronasal mask or tracheal intubation device, this patient valve operating to connect the respiratory passages of the patient to the control unit during an inhalation phase, and to an exhaust outlet during the exhalation phase. Devices of this type in compact portable form are widely available to Emergency Services such as ambulance crews for resuscitation and short-term ventilation purposes, the devices in this form being almost invariably designed to deliver pulses of pure oxygen from a compressed oxygen source because not only is pure oxygen ventilation beneficial for short-term application such as in resuscitation, but its use in this way permits the control unit and patient valve to be very simple and robust devices, resistant to abuse and easily operated correctly by personnel with a minimum of training. Indeed it is these features of such devices that have led to their increasing use for longer term ventilation as when patients with impaired respiratory function are subject to extended transportation. However, medical opinion is not undivided about the merits of long-term ventilation with pure oxygen. Moreover such devices necessarily have a relatively high rate of oxygen usage so that their long-term use can lead to problems of oxygen source availability.
There is therefore a need to provide a compact and portable gas-powered resuscitator/ventilator device with the facility to deliver to a patient, selectively, pure oxygen, oxygen diluted with air, or air. An objective of the present invention is to meet this need but without significantly affecting the standards of robustness, portability, resistance to abuse and ease of operation at present available in devices capable only of delivering pure oxygen.
The type of gas-powered resuscitator/ventilator device here of interest divides into two main sub-types: the sub-type characterised by a low-force patient valve and that incorporates a control unit that generates pulses at a low pressure appropriate for direct delivery to the patient; and the sub-type characterised by a high-force patient valve and that incorporates a control unit that generates either high-pressure pulses or relatively low-pressure pulses that are only a small amount higher in pressure than the pulses to be delivered to the patient. The devices having a low-force patient valve use a low impedance (large bore conduit) connection between the control unit and the patient valve and can be vulnerable to malfunction in the presence of contamination, whereas the devices having a high-force patient valve can have a high impedance (small bore conduit) connection between the control unit and the patient valve. The high-force patient valve has a number of advantages from the point of view of reliability in addition to its ability to operate with a relatively high impedance connection to the control unit. U.S. Pat. No. 4,004,603 and its counterparts disclose a patient valve of this type. For convenience herein, devices having high-force patient valves will be called, shortly, "high-force devices".
Traditionally, in ventilators designed for long-term use in, say, a hospital, dilution of oxygen to a desired breathable gas composition--for instance, a 30/70 oxygen/air mixture having therefore about 45% oxygen content--is accomplished by use of an entrainment mixer. However, the application of such an entrainment mixer to a gas-powered resuscitator/ventilator of the type of interest is not straightforward because such mixers can be designed to give either a high mixing ratio or high pressure recovery, but not both simultaneously. If, therefore, the mixer is designed to achieve the desired oxygen dilution the pressure recovery will generally be insufficient to allow of the use of a high impedance downstream connection: in other words, if the mixer is incorporated in the control unit, the latter will not be able to output pulses able to drive a high-force patient valve through a high-impedance conduit. In this context "high-impedance conduit" is to be understood as meaning a conduit that at the maximum flow therethrough imposes a pressure drop of more than about 3.5 kPa (0.5 psi).
Accordingly, to maintain the advantages of the high-force device sub-type, an entrainment mixer if used to accomplish oxygen dilution must be located at the patient valve. However this leads to control problems because of the basic requirement, in a resuscitator/ventilator device of the type of interest, to be able to deliver pure oxygen when necessary, e.g. for resuscitation purposes. Thus it must be possible to disable the entrainment mixer, or to substitute a patient valve not having such a mixer, to provide for delivery of pure oxygen when this is required.
Selective disablement of an entrainment mixer at the patient valve, or the facility to exchange patient valves with and without entrainment mixers, respectively, leads to flow regulation problems because flow regulation is normally performed within the control unit by means of a high-impedance restriction that makes the output of the unit independent of the impedance characteristics both of the patient valve and of the conduit connecting the latter to the control unit, and also independent of patient compliance, over a wide range of values. If the flow regulation is set to deliver in accordance with the requirements of pure oxygen ventilation then, when a patient valve having an entrainment mixer is brought into operation to achieve dilution of oxygen delivered by the control unit, the oxygen delivery will be in excess of oxygen requirements and wastage will occur. That is to say, while the device in these circumstances delivers diluted oxygen as desired, its oxygen consumption will remain at the rate corresponding to usage of pure oxygen.
Accordingly it would appear that provision must be made at the control unit for resetting the flow regulation in accordance with the absence or presence at the patient valve of an operative entrainment mixer. This is obviously undesirable in a device that must be used, perhaps under adverse emergency conditions, by personnel with little training and/or who are called upon to use the device infrequently.
However, it has been discovered that it is in fact possible to avoid the apparent need for resetting of flow regulation arrangements at the control unit, by giving the patient valve impedance characteristics that affect the flow regulation function, appropriately to reduce oxygen delivery by the control unit, when there is an operative entrainment mixer at the patient valve.